1. Field of the Invention
The present invention relates generally to battery chargers, and more particularly to a charger for safe fast-charging of batteries, which may have widely varying power and charge characteristics, without interfering with logic operations of the charger.
2. Description of the Background
A rapidly growing array of devices utilize nickel-cadmium (NiCd) batteries as a rechargeable, portable power source. A battery's size determines, in large measure, the overall power capacity of the battery. In general, a larger battery will remain charged and able to provide power longer than a smaller battery, when subjected to identical load conditions. Many devices place great demands on their batteries, yet require those batteries to be of relatively small size. In other words, many devices discharge their batteries in a relatively short period of time.
Many such devices are desirable to be kept in substantially constant operation. For example, consider a carpenter who makes almost constant use of a cordless drill operating on a 9.6 volt NiCd battery which fits within the drill's handle. The small size of the battery, coupled with the substantial power requirements of the drill, means that the battery will be discharged in a period of time perhaps as short as one hour or less. While the battery is being recharged, the carpenter cannot work, unless he has a spare, charged battery.
The longer the charging period of the battery, the more spares (and chargers) the carpenter must have on hand. Thus, it is highly desirable to have a battery charger which is capable of fully charging a discharged battery in as short a time as possible. In particular, if the charging period is not longer than the discharging period, the user needs only two batteries-- one in use, and one being charged--and the user needs only one battery charger.
The desired charging time (C.sub.time), the available charge current (I.sub.charge), and the given battery's charge capacity (AH.sub.b) combine to define a "C rating" of the particular charging method: EQU C rating=(C.sub.time * I.sub.charge)/AH.sub.b
Under theoretically ideal conditions, for a battery having a 400 milliampere-hour charge capacity, a 400 milliampere charge current will fully charge the battery in one hour. This is the "C rate" charging method. If only 200 milliamperes of charge current are available, it will take two hours to charge the battery. This is the "C/2 rate" or "0.5 C rate". If the battery must be charged within a half hour, a "2 C rate" must be used, requiring an 800 milliampere charge current.
However, in practice, a C rate charge will not fully charge a battery in one hour, owing to various inefficiencies in the battery and the charger, such as heat generation caused by electrical resistance. The following Table 1 illustrates commonly accepted definitions of C ratings:
TABLE 1 ______________________________________ Charge Method C rating Charge Time (hours) ______________________________________ Trickle C/50 to (used to maintain C/10 fully charged batteries) Standard C/20 36-48 C/10 16-20 Quick C/5 7-9 C/4 5-7 C/3 4-5 Past C 1.2 2C 0.6 3C 0.3 ______________________________________
There are several problems with the "fast" charging of NiCd batteries. First, because internal pressure within a NiCd battery increases as a function of the charge current, if a NiCd battery is charged too quickly, it may explode, causing the loss of the battery, probable damage to the charger, and perhaps great harm to bystanders. Second, because temperature within a NiCd battery increases as a function of the charge current, if the battery is too hot, it will explode. Third, if a NiCd battery is too cold, it will not take a charge. And fourth, attempting to further charge an already fully-charged battery ("overcharging") may also cause harm to the battery, and is wasteful of the electrical supply power.
The user may need a wide variety of NiCd batteries, each with unique power and charging characteristics. For example, the carpenter may use a 9.6 volt battery in a drill, a 1.5 volt battery in a penlight, a 6 volt battery in a handheld calculator, and a 24 volt battery in a motorized shop cart. For the carpenter, it is financially desirable that each type of battery not require its own charger.
It is also financially desirable to the carpenter to reduce wear and tear on the charger and to avoid wasting electrical power. Thus, it is desirable that the charger's most power-consuming components remain in an unpowered or at least "stand-by" state when no battery is present for charging. One possible solution is to simply unplug the charger from its power supply when not in use. This is somewhat inconvenient, and has the drawback that one may forget to plug the charger in when inserting a battery for charging, resulting in down-time for the battery-powered devices which rely on that battery. This solution has the further drawback that one may also forget to unplug the charger after removing the battery, leaving the charger powered and attempting to charge a battery when none is present. This wastes power and reduces the serviceable lifetime of the charger. Another possible solution is to provide an on/off switch, but this is essentially the same as the first solution.
A more elegant solution is to have a battery charger which automatically detects whether there is a battery coupled for charging, and which takes itself into a low power "sleep" mode or stand-by state when no battery is present, or when the battery has become fully charged.